The inventions relates to the field of high temperature applications and concerns an article as well as a method of manufacturing an article which is designed for resisting high temperatures and therefore comprises a protective coating. The invention also relates to a method for refurbishing a gas turbine blade or vane.
In high temperature applications, like running a gas turbine, parts of the used devices are in contact with hot medium and are therefore subjected to oxidation and corrosion as well as decreased mechanical strength.
In a gas turbine, components such as blades, vanes and the like in the hot gas path are in contact with a very hot gas generated in a combustion chamber. The temperature of the hot gas can be above 1000xc2x0 C. or even higher than 1400xc2x0 C. These temperatures exceed the melting point of even specially designed metallic materials. Accordingly, as a typical solution, an inner cooling system is provided in blades or vanes, allowing carry off of heat by a cooling medium like air or steam. Another conventional improvement of heat resistance is to apply a coating for increased oxidation and corrosion protection. A widely used class of oxidation and corrosion protective coatings are MCrAlY coatings. M stands for one or more elements selected from the group Iron (Fe), cobalt (Co) and Nickel (Ni). Cr is Chromium and Al is Aluminum. Y is Yttrium, but can also stand for one or more elements selected from the group of Yttrium and the Rare Earth Elements. An MCrAlY coating is for example described in U.S. Pat. No. 4,880,614. Such a coating may also be used as a bond coat for a thermal barrier coating. A thermal barrier coating is usually a ceramic coating like Yttria stabilized Zirconium-di-oxide. A coating system with a thermal barrier layer on a bond coat is for example disclosed in U.S. Pat. No. 4,321,310.
Such protective coatings are deposited on a metallic base body which could be made from a nickel- or cobalt-base superalloy. These superalloys show outstanding high-temperature strength. A Co-base superalloy is for example known in the art as MAR-M 509 and has a composition comprising carbon, chromium, nickel, tungsten, tantalum, titanium, zirconium, and remainder cobalt. A particular composition, disclosed in U.S. Pat. No. 5,922,150, is in weight percent, of between about 0.54 and about 0.66 carbon, up to about 0.10 manganese, up to about 0.40 silicon, up to about 0.15 sulfur, between about 21.60 and about 26.40 chromium, between about 9.00 and about 11.00 nickel, between about 6.30 and about 7.70 tungsten, between about 3.15 and about 3.85 tantalum, between about 0.18 and about 0.22 titanium, between about 0.45 and about 0.55 zirconium, up to about 1.50 iron, up to about 0.01 boron, and remainder cobalt.
Carbon is of particular importance for the high temperature strength of the superalloy by forming distributed carbides. Therefore, it is known to carburize steels and other carburizable alloys in addition to their bulk carbon content in order to improve the surface hardness. Such a carburization is done by heat treatment in a carbon containing atmosphere. A carburization process is also known from U.S. Pat. No. 6,129,988 for generating carbides in MCrAlY coatings.
As an underlying cognition of the invention, it was discovered that in a region between an oxidation- and corrosion protective coating comprising a carbide forming element and an metallic base body made from an alloy containing carbon, the available carbon from the alloy and the carbide forming element from the coating may interact, resulting in the formation of interfacial carbides below the protective coating. This carbide formation and precipitation causes premature spall and loss of the protective coating.
It is accordingly an object of the invention to provide a high temperature resistant article with a protective coating that has a particular long term bonding property.
It is another object of the invention to provide a method of manufacturing such a high temperature resistant article.
It is a further object of the invention to provide a method of refurbishing a gas turbine blade or vane which results in a particular long term bonding property of a new applied protective coating.
According to the present invention, a high temperature resistant article is provided, comprising a base body, the surface of the base body being at least partly coated with an oxidation- and corrosion protective coating containing a carbide forming element, wherein the base body is made from a metallic alloy having a medium carbon content and wherein the carbon content in a depth of 50 xcexcm or deeper from the coated surface is less than 0.3% of said medium carbon content.
I.e., the base body has a low carbon zone near the surface. Accordingly, carbide formation is reduced or even nearly completely prevented because the carbide forming elements from the protective coating can not interact with carbon from the alloy or at least the interaction is reduced. As a consequence, carbide formation and precipitation between the protective coating and the base body decreases which results in an extended service life because of a decrease in spallation tendency of the protective coating. However, the high temperature strength of the base body is not vitiated because the bulk carbon content remains sufficiently high at an original intended level.
The protective coating contains preferably cobalt and is even more preferred of the type MCrAlY, with M being an element selected from the group (Iron, cobalt, Nickel) or a mixture thereof, Cr being Chromium, Al being Aluminum and Y being Yttrium or a Rare Earth element or combinations thereof. The protective coating may optionally further include elements such as Rhenium and the like, as well as phases such as oxides, as will be understood by those skilled in the art.
The alloy contains preferably cobalt and is preferably a nickel- or cobalt-base superalloy with the even more preferred composition MAR-M-509.
The article is preferably a gas turbine blade or vane. According to another aspect of the invention, there is provided a method of manufacturing a high temperature resistant article, the method which comprises the following steps:
manufacturing a base body from a metallic alloy having a medium carbon content;
decarburizing a surface of said base body, thereby decreasing said medium carbon content near said surface such that in a depth of 50 xcexcm or deeper from said surface a content of carbon is less than 0.3% of said medium carbon content;
coating said surface with an oxidation- and corrosion protective coating.
The advantages of this method of a selective reduction of carbon in the surface of the base body correspond to the above described advantages of the high temperature resistant article.